The most important European war in history was the Seven Years' War (1754-1763), of which I will mention only that it ensured North America and India would be under the rule of England. World War Two settled virtually nothing; World War One settled a lot, but by 1992 its effects had been completely erased. The Thirty Years War (1618-1648) was important insofar as its demographic and economic effects were stupendous, but in terms of the fate of the world, it's hard to surpass the Seven [actually, Nine] Years' War.
This most important of conflicts ended with a conference held in Paris; unlike other major historical conflicts, it ended with a bargain. Prussia retained Silesia, enabling the junker class of that country to avail themselves of the economic and industrial resources to create Germany as it would become. Russia, whose armies had nearly liquidated Prussia forever, suddenly had a change of government, and withdrew from the war. This was another crucial step in a sort of protracted divorce between Peter's Russia and Western Europe. France was compensated for the loss of Canada and India with the far-more lucrative possessions of Reunion, Guadeloupe, and Martinique; and Spain lost Florida (to Great Britain) but was compensated with Louisiana and parts west.
This War was followed, as is well known, by increasing confidence on the part of the settled farming and mercantile populations of North America; strategic security led to political assertiveness, thence to acrimony with the English Crown, thence to a revolution in these colonies and a concomitant war with the King of England. Again, the war was settled with a peace conference in Paris (1783); Britain was compensated for the loss of the United States with the far more lucrative colony of Gambia (one of several vital contributions the Bourbons made to the creation of the USA). A mere five years late came the French Revolution, then the First Republic, the Coups of Napoleon, and the Napoleonic Wars (1792-1815).
The Napoleonic Wars were an immensely complex family of conflicts, in which virtually every nasty issue of European politics was dragged out, flogged, and kicked back under the carpet. Understandably, most books about the Napoleonic wars tend to speak of them as having been resolved by the Congress of Vienna, when in fact they were resolved by many diplomatic conferences and agreements.
Treaty of Paris (1814): This was the immediate resolution of the war with France: restoration of the borders of 1792, restoration of the Bourbon monarchy, and no indemnity. Tobago, St. Lucia, and Mauritius (all big money-makers) were retained by England. This astonishingly lenient settlement owed much to the highly personal cabinet politics of the day—and the brilliant diplomacy of Talleyrand.
Congress of Vienna (1814-15): the Napoleonic Wars had irreversibly altered the political organization of Germany and Italy; the Holy Roman Empire, for example, had been liquidated and in large measure replaced by the Confederation of the Rhine. The princes whose dominions had ben united to create the C.o.t.R. were also beneficiaries of huge increases in their territory, and as such their permission was required for any postwar settlement. So the entire project of a German federation from Carniola to Pommerania was abandoned in favor of conventional monarchies. Russia, Prussia, and Austria divided up Poland (again), and Italy was partitioned among the three most reactionary monarchies of Europe. The former Austrian territories in Belgium were awarded to the Netherlands, foreshadowing the modern federation of Benelux.
Congress of Aix-la-Chapelle: in 1818; negotiation of the withdrawal of allied forces from France and the possibility of a permanent counterrevolutionary alliance of the Five; UK opposition is the usual reason given for the failure, although it needs to be noted that the actual business conducted in secret at Aachen was directed towards uniting the four wartime allies against potentially resurgent French expansionism. This, of course, was in direct contradiction to the spirit of unity that the Congresses were supposed to have been promoting. On the other hand, had either Hardenberg (Prussia) or Metternich (Austria) assumed the thankless task of saying no to Czar Alexander's schemes, they would have faced a possible reprisal. Only the UK was entirely beyond the reach of Romanov retaliation. For this reason, secret communications among the intermediaries may well have orchestrated a wished-for failure.
Congress of Troppau: The Quadruple Alliance that had supposedly emerged from the Congress of Vienna, was actually just an expression; Talleyrand had successfully split it, to enable a moderate settlement and equality of status during the Congress of Vienna. Troppau, incidentally, was then the capital of Austrian Silesia; it is now known as Opava, in the Czech Republic, right on the border with Poland. Here, in 1820, the representatives of Austria, Russia, Prussia, Great Britain, and France discussed a scheme of mutual defense of "legitimist" regimes. Austria, Russia, and Prussia signed a protocol (proposed by Alexander I) that threatened armed action against any revolutionary attempts to disturb the status quo. Britain and France refused to adhere to the protocol, marking the first serious weakening of the congress system.
Congress of Laibach: Laibach is the old name for a really beautiful town now known as Ljubljana, Slovenia; I am probably obligated to remind readers that Slovenia is the country south of Austria, and must not be confused with Slovakia (Slovenska), the larger country to the northeast of Austria. The most notable event was the failure of Britain and the three eastern powers to agree on a general rapid reaction (and we do mean "reaction") force. Austria and the two pathetic landlords of Italy (viz., the Savoyards of Piedmont-Sardinia and the Bourbons of Naples-Sicily) cut a deal to crush the Italian patriots, however.
Congress of Verona: This was the last meeting of the Quadruple Alliance; the British, again, rejected a reactionary intervention in Spain, while the Austrians, Prussians, and Russians approved it.
ADDENDUM: Someone in comments has asked me to describe (a) the national unification projects of the 19th century (Italy and Germany). Also, (b) how Germany was transformed from independent principalities into a police state under Prussian dominance.
(a) There is an abundance of material on the internet regarding this matter, and I don't want to duplicate any more effort than I have to. With respect to Germany: the core state of this country was Brandenburg, plus some Rhenish territories belonging to the Hohenzollerns. I believe, but perhaps am mistaken, that the merger of Brandenburg with Prussia (1618) and some other Pommeranian territories (1648), altered the character of Brandenburg. After annexing Silesia (1740), it was to become dominated politically by junkers, or landlords belonging to a military caste. This was to constitute the backbone of the Prussian state, even though revenues from the western, bourgeois, part of Prussia were greater. The junkers were the absolutely essential patrons of the Hohenzollern, whereas the burghers had an standing deal with the Hohenzollerns. The west was the client of Berlin; the east was its spouse.
(For some discussion on the junkers and their immense importance, please see the Max Weber essay, "National Character and the Junkers.")
The French Revolution, as I mentioned above, led ultimately to invasions of the French homeland, and the French mobilizing to carry the war to their attackers. One of the effects of this was Napoleon's consolidation of the territories of Western Germany, either by awarding early acceptors of French rule with the lands of their neighbors, or else secularizing the ecclesiastical states. Fory-five (of 51) imperial cities (i.e., cities autonomous within the HRE) plus 67 other principalities were dissolved in this way and assigned to Baden, Prussia, Württemberg, Berg, and Bavaria. Prussia & Saxony, briefly aligned with France, switched sides from Napoleon to Russia in 1806-07 and got badly stomped. Napoleon therefore annexed the Western territories of Prussia (see map below).
The collapse of France in 1814 and the Congress of Vienna restructured Germany again. Prussia was restored, including territories awarded earlier to Russia. The German princely states began to integrate industrial infrastructure and customs long ahead of the 1848 revolutions; the latter forced several kingdoms, like Bavaria, to adopt a constitution restricting the power of the king. However, the 1849 project to draft a constitution for the whole of Germany (which would have included Austria) was scuppered by the Prussian King ("I will not accept a crown from the gutter").
Things began to move quickly in 1864: first, a long-simmering border dispute with Denmark lead to a second war [1st war, 1848], with Prussia and Austria seizing the territory; then, the Seven Weeks' War in which Austria and Prussia came to blows over the division of the spoils (and in which Austria's allies, Hanover and Hesse-Kassel, were liquidated and annexed to Prussia; then, in 1870, Napoleon III decided to go to war with Prussia. The French campaign was an utter disaster, Napoleon was actually captured, and France declared for the 3rd Republic. The Republican commune in Paris renewed the declaration of War and German forces beseiged Paris through 1871; soon after, radical communards in Paris wage a civil war with the conservative Versailles government.
The resumption of hostilities with a defeated France led many Germans to believe that France was their "eternal enemy" (Erfeind), and that unification under Prussia was necessary. Hence, the unification of Germany occurred under condition of national outrage and temporary military expedience.
Italy: The two landlords of Italy to whom I referred above were restored, but they were now effectively known as foreign agents, since they ruled entirely at the behest of major powers and accepted the hugely unpopular occupation of n.e. Italy by the new Austro-Hungarian Empire. Austria had, in fact, been awarded those territories as the result of a deal cut with Napoleon in 1797, so the Italian conservatives also had good reason to resent the Austrian overlords. This intensified after the Laibach-authorized Austrian invasion of Italy to crush a liberal uprising against the Bourbon rulers of Naples (1820). King Ferdinand II started out as a liberal, but like most such leaders promptly became obsessed with defeating the left and in 1848 suffered a constitutional revolution; he dithered until the Austrians won a victory nearby, then violated his oath to the constitution, beseiged the major cities, and shelled them for eight hours after they capitulated.
The Kingdom of Piedmont-Sardinia had waged a war against the Austrians occupying n.e. Italy; this effort was what the Austrians had defeated in 1848. Carlos Alberti of PS abdicated in favor of his son, Vittorio Immanuel I, who colluded with Napoleon III to defeat Austria in 1859, and annex the Kingdom of the Two Sicilies upon the death of Ferdinand II. Also important was the expedition of Giuseppe Garibaldi, in which he lead a victorious revolution against the reactionary monarchy. In this way, the two main kingdoms of the 2 Sicilies and PS were united under the Savoyards. Rome, however, remained in the Papal States, protected by the forces of Napoleon III.
In 1866, as Austria went to war with Prussia (see above), Italy allied with Prussia and won control over Venezia; then, in 1871, as French troops retired from Rome, Italian troops arrived. Pope Pius IX, the supreme leader and ideogue of 19th century landlord reaction, was a prisoner of the Vatican.
Columbia Encyclopedia: Treaties of Paris (several of them!); Vienna, Congress of; Troppau, Congress of; Verona, Congress of; Laibach, Congress of; recollections of books, like A Short History of British Expansion, by Williamson & Southgate, and a long bio of Talleyrand I read about 11 years ago.